The present invention relates to a pulse doppler measuring apparatus, and more particularly to an apparatus for detecting the speed of a moving object by using an ultrasonic wave, for example, a pulse doppler measuring apparatus capable of measuring the flow speed of blood in a living body in realtime with a high signal-to-noise ratio.
Various kinds of apparatuses have hitherto been known which detect the flow speed of an object by utilizing the Doppler effect of an acoustic wave. Specifically, in an apparatus using the pulse Doppler method which is described in, for example, an article entitled "Pulsed Ultrasonic Doppler Blood Flow Sensing" by D. W. Baker (IEEE Trans. Vol. SU-17, No. 3, July 1970 pages 170 to 185), a pulsed continuous wave is sent out repeatedly, and a time gate corresponding to the distance to a measured part is set on a received signal to specify the measured part.
An ultrasonic Doppler blood flow measuring apparatus has been known, in which, as disclosed in, for example, JP-A-58-188433, JP-A-60-119929 and JP-A-61-25527, an ultrasonic wave is transmitted toward a blood vessel, and the Doppler shift frequency of the ultrasonic wave reflected from the blood in the blood vessel is measured to detect vcos.theta., where .theta. represents an angle between the direction of blood flow and the transmission direction of the ultrasonic wave, and v indicates a blood flow speed.
Further, a technique called "color flow mapping", in which the distribution of blood flow speed in a cross section of a living body is measured and displayed in color on a tomographic image, is described in an article entitled "Real-Time Two-Dimensional Blood Flow Imaging Using an Autocorrelation Technique" by C. KASAI et al. (IEEE Trans. Vol. SU-32, No. 3, May 1985 pages 458 to 464). In order to carry out the color flow mapping at a desired image frame rate, the blood flow speed at each of a plurality of pixels is determined by averaging the measured values of Doppler shift due to a relatively small number of measurements. In the example mentioned in the above article, a difference vector between a vector indicated by a Doppler signal detected currently and a vector indicated by the preceding Doppler signal is obtained by an autocorrelator for each of the measurements, and the average speed is calculated from the argument of a vector which represents the sum of a plurality of difference vectors. That is, the autocorrelation method is used in the above example.
Meanwhile, U.S. Pat. No. 4,809,703 discloses the so-called two axial component method, in which a phase difference .DELTA..theta. of a Doppler signal obtained for each measurement is decomposed into a cosine component and a sine component, a plurality of values of each of the cosine and sine components are added and averaged, and a phase difference indicated by the average cosine and sine components thus obtained is transformed into a velocity.
Further, an article entitled "Blood Flow Imaging Using a Discrete-Time Frequency Meter" by M. A. Brandestini and F. K. Forster (1978 Ultrasonics Symposium Proceedings pages 348 to 352) shows a method in which the phase difference of a Doppler signal is detected for each of a plurality of repetitions of measurement, and an average phase difference is calculated by adding a plurality of values of phase difference directly, to be converted into a velocity. This method will hereinafter be referred to as "phase difference averaging method".
Meanwhile, it is pointed out in U.S. Pat. No. 4,905,206 that the phase difference averaging method produces a large calculation error when a true average phase difference is close to +.pi. or -.pi., that is, a moving object is put in a high-speed region, and that the autocorrelation method and the two axial component method produce a large calculation error when the true average phase difference is close to zero, that is, the moving object is put in a low-speed region. U.S. Pat. No. 4,905,206 further discloses that one of the phase difference averaging method and the autocorrelation method (or the two axial component method) can be changed over to the other so that the above difficulties are eliminated, and that values of phase difference obtained for a plurality of measurements are transformed into those in a new polar coordinate system using a direction which is indicated by the average phase difference angle according to the autocorrelation method, as a reference axis, and the values of phase difference thus obtained are added and averaged.